Even identical twins - who have the same genetic code - differ in whether they are left-handed or right-handed.

The new research says that nurture plays a decisive factor.

For around one in five left-handers, the key reason for their dominant handedness is whether they were breast fed or not.

The study was based on research in five countries - including in the United States, the United Kingdom, and Ireland - involving 62,129 pairs of mothers and babies.

Being bottle-fed as a baby makes you more likely to be left-handed, research has found. For around one in five left-handers, the key reason for their dominant handedness is whether they were breast fed or not (stock image)

Breastfeeding for more than six months overall decreased the chance of any baby being left handed from 13 per cent to 10 per cent, the researchers said.

This means that around 22 per cent of left-handers would be right handers if they had been breast fed for six months or more, rather than bottlefed.

The researchers suggest that the process where the right side of the brain and left side of the brain take on specialist functions takes place up to nine months.

Advantages for babies who are breastfed by healthy mothers include increased intelligence, increased head circumference, decreased speech problems, and decreased multiple sclerosis, the researchers said.

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Philippe Hujoel, the study’s author and dentistry professor at the UW School of Public Health said: ‘We think breastfeeding optimizes the process the brain undergoes when solidifying handedness.

‘That is important because it provides an independent line of evidence that breastfeeding may need to last six to nine months.’

Possible causes include the nutritional content of breast milk versus formula milk having an effect, or hormones produced in the baby when in contact with the breast.

The study was based on surveys in five countries, including the United Kingdom and Ireland involving 62,129 pairs of mothers and babies.

Researchers at University of Washington School of Public Health studied g 62,129 pairs of mothers and babies in five countries - including in the United States, the United Kingdom, and Ireland (stock image)

The authors write: ‘Breastfeeding for more than 6 months, when compared to bottle feeding, was associated with an absolute 3 per cent decreased prevalence for nonrighthandedness, from 13 per cent to 10 per cent, respectively.

‘Twenty-two percent, or about one in five cases of non-righthandedness among bottle-fed infants may thus be attributable to a lack of breast feeding for more than 6 months.’